Apparatus for stretch forging and apparatus for forming the same

ABSTRACT

Apparatus is provided for stretch forging having swinging jaws depending from driven eccentrics and offset relative to one another around the run-through axis of an ingot workpiece. Means are provided for rotating the ingot through a predetermined angle about its axis. The jaws form in closed position a closed sizing gap in the region of the neutral flow plane of the workpiece, and the configuration of the working surface of each jaw being such that a portion of the workpiece is radially deformed.

United States Patent 1191 [111 3,91 1,720

Kocks Oct. 14, 1975 [54] APPARATUS FOR STRETCH FORGING AND 3,626,746 12 1971 Pietryka 72 406 APPARATUS FOR FORMING THE SAME FOREIGN PATENTS OR APPLICATIONS Inventor: Friedrich Kocks, Freiligrathstrasse 518,968 4/1953 France 72/189 1, 4 Dusseldorf, Germany [22] Fil d; D 7, 1973 Primary ExaminerC. W. Lanham Assistant ExaminerGene P. Crosby [211 App]. No.: 411,500

Related US. Application Data Continuation-in-part of Ser. No. 154,749, June 21, 1971, abandoned.

[52] US. Cl. 72/406 [51] Int. Cl. B21J 9/18 [58] Field of Search 72/406, 189, 402, 470, 72/475, 404

[56] References Cited UNITED STATES PATENTS 2,153,839 4/1939 Liebergeld 72/189 3,114,276 12/1963 Uebing 72/406 3,374,654 3/1968 Uebing.... 7 2/189 3,583,192 2/1969 Kocks 72/406 Attorney, Agent, or Firm-Buell, Blenko & Ziesenheim [57] ABSTRACT Apparatus is provided for stretch forging having swinging jaws depending from driven eccentrics and offset relative to one another around the run-through axis of an ingot workpiece. Means are provided for totating the ingot through a predetennined angle about its axis. The jaws form in closed position a closed sizing gap in the region of the neutral flow plane of the workpiece, and the configuration of the working surface of each jaw being such that a portion of the workpiece is radially deformed.

4 Claims, 15 Drawing Figures US. Patent Oct. 14,1975 Sheet10f9 3,911,720

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J Friedrich Kock m g N 3 lnvehtor Frigdrich Kocks "iag Inventor:

Friedrich Kocks fiAfi gwh US. Patent Oct. 14, 1975 Sheet 4 of9 3,911,720 Q lnv ntor;

Friedrich Kocks US. Patent Oct. 14, 1975 Sheet 5 of9 3,911,720

B/ T1 A T lnventof: Friedrich Kocks US. Patent 00:. 14, 1975 Sheet 7 of9 3,911,720

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Friedrich Kocks Sheet 8 of 9 Oct. 14, 1975 US. Patent US. Patent Oct. 14, 1975 Sheet 9 of 9 3,911,720

APPARATUS FOR STRETCH FORGING AND APPARATUS FOR FORMING THE SAME This application is a continuation-in-part of my copending application Ser. No. 154,749, now abandoned, filed June 21,1971. 7

The invention relates to apparatus for stretch-forging and apparatus for forming the same, particularly to an apparatus having swinging jaws which are offset relative to one another around the run-through axis of the ingot, each of which jaws depends from two drivable crank devices or eccentric devices for stretch forging metallic materials.

In order to be able to produce tubes of good quality from hollow ingots or blooms by means of such an apparatus, together with a great reduction in crosssection, it has previously been proposed in U.S.- application Ser. NO. 114,829, now U.S. Pat. No. 3,728,890, that all the swinging jaws be simultaneously moved towards and away from the ingot and simultaneously moved forwardly and backwardly in the feed direction. Preferably, the ingot to be processed is turned through a predetermined angle about its longitudinal axis each time the swinging jaws are raised.

A feature of the present invention is to further improve the subject of the parent application such that the quality of the workpiece is-further improved, together with a reduction in the cost of tools.

In accordance with the invention, swinging jaws when in their moved-together state form a closed sizing gap in the region of the neutral flow plane of the workpiece, and the configuration of the working surface of the swinging jaws in the region of the deforming part is such that it is possible to broaden the workpiece, at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through axis of the workpiece.

Since it is to be possible freely to broaden the workpiece, the radius of curvature of the working surface of the swinging jaw in the region of the deforming part can, for example, be greater in the section at right angles to the run-through axis of the workpiece than the shortest distance between the centre of the sizing gap and the corresponding point on the working surface, or the radius of curvature may be equal to the distance between the centre of the sizing gap and the working surface, the rim region then having to be flattened on each side of the working surface in the region of the deforming part.

The arrangement and configuration of the jaws in accordance with the present invention results in a particularly favorable flow of material in the workpiece and thus in a product of good quality. At the sametime, there is the advantage that the working surfaces of the swinging jaws can be produced very simply by means of a suitable device by virtue of the arcuate crosssectional configuration and the course of movement in accordance with the invention in which a defined, narrowest sizing gap cross-section is achieved in all positions of the jaws during the forming cycle.

A device, constructed in accordance with the invention, for machining the swinging jaws comprises a holding device holding a swinging jaw to be machined and for imparting to the jaw the same movement as is to be imparted to the jaw in the stretch-forging apparatus, a rotatable turning chisel arranged opposite to the working surface of the jaw, means for advancing the chisel in synchronism with the movement of the swinging jaw, the working position of the chisel corresponding to the region of the neutral flow plane of the workpiece, subsequently to be elongated, below the rolling jaw, and means for radially adjusting the turning chisel during the machining operation in conformity with the shape of the jaw to be produced.

An axially displaceable control pin may be coupled to the turning chisel for radially adjusting the turning chisel and may be guidable parallel to the advance movement of the turning chisel along a template conforming to the longitudinal section of the jaw profile to be produced.

Several or all the swinging jaws of one set may be ma chined simultaneously in such an apparatus, the turning chisel rotating in the region of the calibrating gap subsequently closed during operation. During machining, the distance between the jaws has in each case to be adjusted in conformity with the radius of the working surfaces to be produced.

The invention is further described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of the basic construction of a stretch-forging apparatus shown in four successive working positions in which the swinging jaws are rolling on the workpiece;

FIGS. 2 to 5 are sections taken on the lines 11-11, III- III, [VIV and VV respectively in FIG. 1, the sections being associated with the successive positions of the jaws;

FIGS. 2a to 5a are sections corresponding to those of FIGS. 2 to 5 of a different embodiment of the swinging jaws;

FIG. 6 shows a detail from FIG. 2, drawn to a larger scale;

FIG. 7 is a detail from FIG. 2a, also drawn to a larger scale;

FIG. 8 is a diagrammatic illustration of the construction of a device, constructed in accordance with the invention, for machining the swinging jaws illustrated in FIGS. 2 to 6;

FIG. 9 is a diagrammatic illustration of a device for machining the swinging jaws illustrated in FIGS. 2a to 5a and 7;

FIG. 10 is a partial lengthwise section through one form of the device of this invention;

FIG. 11 is a cross section transverse to the line of feed of a device of this invention with work in place.

Referring to the drawings, stretch-forging apparatus constructed in accordance with the invention has four swinging jaws 2 which are offset by relative to one another and which are arranged around the runthrough axis 4 of the workpiece which comprises a hollow ingot 6. The four swinging jaws 2 simultaneously roll with uniform kinematics on the workpiece and reduce the hollow ingot 6, mounted on a mandrel 8, to form a tube which may subsequently be fed to a stretch-reducing mill.

The movement of the individual swinging jaws 2 is carried out by means of a drive of the kind illustrated and described in German Patent specification Ser. No. 1,298,495, Uebing et al U.S. Pat. No. 3,114,276, Uebing et al U.S. Pat. No. 3,374,654 and Kocks U.S. Pat. NO. 3,728,890, for example, in which the swinging jaw drive comprises two eccentrics 10 and 12 from which a respective swinging jaw depends by way of links 14 and I6 pivotable in the plane of the feed direction. The swinging jaw is mounted in a holder 18 to which the links 14 and 16 are secured by way of respective pivots 20 and 22. During movement of the jaw, the pivots 20 and 22 move along the respective curved paths 21 and 23 illustrated by dash-dot lines in FIG. 1. Furthermore, the feed movement is assisted by an additional link 26 which is drivable by way of an eccentric 24 and which is pivoted to the end of the link 16 which extends beyond the eccentric 12. During operation, the pivotal point 28 between the link 16 and the additional link 26 moves along the curved path 30 illustrated by dash-dot lines in FIG. 1.

The eccentrics 10, 12 and 24 for all the swinging jaws are coupled together so that they rotate in synchronism whereby all of the jaws are movable simultaneously towards and away from the workpiece and are all simultaneously reciprocable in the feed direction.

When in the moved-together state, i.e. during the rolling or forming movement on the workpiece 6, the four swinging jaws form a closed sizing gap in the region of the respective neutral flow planes of the workpiece, as is shown in FIGS. 2 to and 2a to 5a and 11. At the instant in which the jaws are applied to the workpiece, the closed sizing gap is located on the input side of the jaws and then moves up to the delivery cross-section of the jaws in conformity with the rolling movement of the jaws.

The configuration of the working surface of the swinging jaws in the region of the deforming part is such that it is possible to broaden the workpiece.

Referring to FIGS. 2 to 6 of the drawings, the working surface 32 of the swinging jaws 2a is of arcuate construction in the plane lying at right angles to the runthrough axis 4 of the workpiece 6, the radius of curvature R being greater than the shortest distance A between the run-through axis 4 of the workpiece, or the centre of the sizing gap and the working surface 32. Consequently, the closed sizing gap of the jaws 2a is not circular but, as shown particularly in FIG. 6, is in the shape of a square having greatly bulging sides. The workpiece is turned through 45, i.e. half the angle at which the jaws are disposed relatively to one another, after each stroke of the jaws, so that the apex of the bulged square cross-section is located in the bottom of the sizing gap at each following stroke of the jaws, while there remains in the boundary region between two jaws a space into which the workpiece may be pressed and thus broadened when the jaws move together. This may be accomplished using a hydraulic piston 6a rotating a cam lock rotator 6b of known form which groups and rotates the workpiece through 45 each time the piston 6a is energized. The sizing gap becomes increasingly circular towards the delivery crosssection of the jaws, so that, in this region, only smoothing of the workpiece takes place and scarcely any deformation.

Referring to FIGS. 2a to 5a and 7, the working surface 34 of the jaws 2b has a radius of curvature R in the section at right angles to the run-through axis 4 of the workpiece, the said radius of curvature being equal to the distance between the centre of the sizing gap or the run-through axis 4 of the workpiece 6 and the working surface 34. However, this curvature extends only across a central region B in the deforming part of the jaws, while the rim region on both sides of the working surface is flattened. As is shown particularly in FIG. 7, respectively plane regions T, and T tangentially adjoin the curved region B on each side. As is shown in FIGS. 2a to 5a, satisfactory broadening of the workpiece is possible even when using jaws of such configuration when the workpiece is turned through 45 after each stroke of the laws. The flattened portions T, and T terminate in the so-called smoothing portion at the end of the jaws, so that, as may be seen from FIG. 5a, the sizing gap immediately in front of the delivery cross-section is circular.

The respective working surfaces 32, 34, may be machined very simply in the case of the described swinging jaws 2a and 2b. The arcuate region of the working surface at right angles to the run-through axis 4 of the workpiece 6 may be machined by means of a rotating chisel for example. For this purpose, the swinging jaw to be machined is clamped in a holding device 36 which imparts to the jaw the same movement as is imparted to the jaw in the stretch-forging apparatus. This is the equivalent to the drive for one of the jaws shown in FIGS. 10 and 11. The drive device of the holding device 36 is equipped with the same drive elements as the stretch-forging apparatus and, in the same manner as the latter, has two links 14 and 16 which are drivable by eccentrics 10 and 12, and an additional link 26, which impart corresponding curved paths 38 and 40 to the pivotal points. As may be seen from FIGS. 8 and 9, a chisel 46 which is rotatable about an axis of rotation 42 or 44 is located opposite the working surfaces 32 and 34 of the respective swinging jaws 2a and 2b be machined. The turning chisel 46 is mounted in a shaft 48 whose axis lies in the axis of rotation 42 or 44 of the turning chisel 46 and is displaceable co-axially of the axis of rotation.

The turning chisel 46 is axially displaceably mounted in a transverse bore 50 provided in the shaft 48, the centre axis 52 of which bore 50 intersects the axis of rotation 42 or 44 of the shaft 48 and thus of the chisel 46. The shaft 48 is journalled in bearings 54 and 56 which permit axial displacement as well as rotation. The rotation of shaft 48 and parallel shaft 62 is coupled to drive shaft 22 for the eccentrics l0 and 12 by means of level gears and shafts 80, 81, 82, 83, 84, 85, 86, 87 and 88 are schematically illustrated in FIG. 8. At the same time shafts 48 and 62 are moved back and fourth in union by means of levers 90, 91 and 92 connected to eccentric 93 on drive gear 24 for the jaw.

The shaft 48 effects the feed movement of the turning chisel 46. The axial movement of the chisel 46, directed in the direction of the axis of rotation 42 or 44, is coupled to the rotary movement of the eccentrics l0 and 12 by way of the drive linkage illustrated so that the turning chisel 46 machines the entire length of the swinging jaw 2a and 2b during a rolling movement of the swinging jaw, the working position of the chisel in each case corresponding to the region of the neutral flow plane of the workpiece subsequently located at this position.

Since the swinging jaws in the apparatus constructed in accordan e with the invention continuously approach the run-through axis of the workpiece during the stretchc-forging operation, the continuous closed sizing gap becoming smaller towards the delivery end of the jaws, the turning chisel 46 machining the jaws must remove from the material of the jaws an arc having a continuously decreasing radius of curvature towards the end of the jaw. For this purpose, the turning chisel 46 also carries out an axial adjustingmovement together with its cutting and feed movement. This adjusting movement is effected by means of a control pin 60 coupled to the turning chisel 46. The control pin 60 is mounted in a control shaft 62 lying parallel to the shaft 48 carrying the chisel. The control shaft 62 is journalled in bearings 64 and 66 and is drivable in synchronism with the feed movement of the chisel shaft 48 by means of drive linkage illustrated in FIG. 8 of the drawings. During the feed movement, the control pin 60 axially displaceably mounted in the control shaft 62 is guided along a template 70 whose configuration is adapted to the orbital movement of the working surface of the jaw. Control pin 60 acts through a hollow passage 100 at the axis of the shaft 62 to control the fluid in piston 101 which actuates serve piston 102 to actuate control piston 103 connected to passage 106 in shaft 48 which in turn is connected to bore 52 for adjusting chisel 56.

For the purpose of manufacturing swinging jaws 2a illustrated in FIGS. 2 to 6, the axis 42 of the chisel shaft 48 slopes relative to the swinging jaw 2a to be machined at an angle towards the imaginary run-through axis 4 of the workpiece or through the axis through the centre of the sizing gap, the distance-between the axis 42 of rotation of the shaft 48 and the working surface 32 to be machined on the jaw 2a being greater on the input end of the jaw 2a than the distance between the working surface and the imaginary centre of the sizing gap, and being equal to this distance at the delivery end of the jaw.

For the purpose of machining the swinging jaws 212 shown in FIGS. 2a to 5a and 7, the axis of rotation 44 of the chisel shaft 48 lies in the centre of the pass in all working positions of the chisel 46 relative to the swinging jaw 26 to be machined, i.e. the axis of rotation 44 is located exactly in the run-through axis 4 of the workpiece subsequently to be processed. Of course, in the embodiment illustrated in FIG. 9, a further machine (not illustrated in the drawings), such as a milling machine has to be provided for flattening the side edges of the working surface. The flattened edge portions are designated T and T in FIG. 7 and may be produced by means of a milling cutter subsequently applied.

Since the operation of milling cutters has been known for decades, it is not deemed necessary to illustrate such milling cutter. Several or all four jaws of a machine chisel in such machining apparatus.

In the foregoing specification I have set out certain preferred embodiments and practices of my invention, however, it will be obvious that this invention can be otherwise embodied within the scope of the following claims.

I claim:

1. Stretch-forging apparatus comprising a plurality of swinging jaws which are offset relative to one another around the run-through axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that a portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through-axis of the workpiece, each said swinging jaw having a working surface with a radius of curvature in the crosssection at right angles to the run-through axis of the workpiece in the region of the deforming part greater than the shortest distance between the centre of the sizing gap and the working surface.

2. Stretch-forging apparatus comprising a plurality of swinging jaws which are offset relative to one another around the run-through axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that a portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through axis of the workpiece, each said swinging jaw having a working surface with a portion of radius of curvature in each cross-section at right angles to the run-through axis of the workpiece equal to the distance between the centre of the sizing gap and the corresponding point on the working surface at that cross-section, and the edge region of the working surface on each side of the radiused portion of the working surface being flattened in the region of the deforming part.

3. Stretch-forging apparatus comprising at least three swinging jaws which are offset relative to one another around the run-through axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through axis of the workpiece, the drives for all the swinging jaws being so coupled together that all of the jaws are movable in synchronism, each said swinging jaw having a working surface with a radius of curvature in the crosssection at right angles to the run-through axis of the workpiece in the'region of the deforming part greater than the shortest distance between the centre of the sizing gap and the working surface.

4. Stretch-forging aparatus comprising at least three swinging jaws which are offset relative to one another around the run-through axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that a portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through axis of the workpiece, the drives for all the swinging jaws being so coupled together that all of the jaws are movable in synchronism, each said swinging jaw having a working surface with a portion of radius of curvature in each cross-section at right angles to the run-through axis of the workpiece equal to the distance between the centre of the sizing gap and the corresponding point on the working surface at that cross-section, and the edge region of the working surface on each side of the radiused portion of the working surface being flattened in the region of the deforming part.

l i l UNITED STATES PATENT QFFICE TIHCATE OF COECTWN PATENT NO. 3 I 911 720 DATED October 14 1975 INVENTOR(S) FRIEDRICH KOCKS It is certified that error appears in the ab0veident|fied patent and that satd Letters are hereby corrected as shown below:

[SEAL] Column 1, line 18, 'Ser. NO. should read --Ser. r:

(second occurrence) Column 2, line 65, "NO" should read --I to.-.

Column 4, line 1, respective.

"respectively" should read Column 4, line 30, after "2b" insert to-.

Column 4, line 44, "are" should read -as.

Column 4, line 62, "stretchc-forging" should read -stretch-forging-,

Column 5, line 47, after "Several or all four jaws of a" and before "machine" insert the following:

unit may be machined simultaneously with the same turning--.

Column 6, Claim 3, line 53, after "that" insert -a-.

Evigncd and Scaled this twenty-third Day of March 1976 Arrest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN (nmmissiumr oj'latents and Trademarks 

1. Stretch-forging apparatus comprising a plurality of swinging jaws which are offset relative to one another around the runthrough axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that a portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through-axis of the workpiece, each said swinging jaw having a working surface with a radius of curvature in the cross-section at right angles to the run-through axis of the workpiece in the region of the deforming part greater than the shortest distance between the centre of the sizing gap and the working surface.
 2. Stretch-forging apparatus comprising a plurality of swinging jaws which are offset relative to one another around the run-through axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that a portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-thrOugh axis of the workpiece, each said swinging jaw having a working surface with a portion of radius of curvature in each cross-section at right angles to the run-through axis of the workpiece equal to the distance between the centre of the sizing gap and the corresponding point on the working surface at that cross-section, and the edge region of the working surface on each side of the radiused portion of the working surface being flattened in the region of the deforming part.
 3. Stretch-forging apparatus comprising at least three swinging jaws which are offset relative to one another around the run-through axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through axis of the workpiece, the drives for all the swinging jaws being so coupled together that all of the jaws are movable in synchronism, each said swinging jaw having a working surface with a radius of curvature in the cross-section at right angles to the run-through axis of the workpiece in the region of the deforming part greater than the shortest distance between the centre of the sizing gap and the working surface.
 4. Stretch-forging aparatus comprising at least three swinging jaws which are offset relative to one another around the run-through axis of a workpiece, each of which jaws depending from two drivable crank devices or eccentric devices moving the jaws toward and away from the axis of said workpiece, means engaging said workpiece by which said workpiece is rotatable through a predetermined angle about its axis, the swinging jaws when in their moved-together state forming a closed sizing gap around the run-through axis of the workpiece whereby the workpiece is substantially completely enclosed within the jaws and means in the configuration of the working surface of each swinging jaw in the region of contact with the workpiece such that a portion of the workpiece is radially deformed within the jaw at least a portion of the working surface of the swinging jaws being of arcuate construction in the plane lying at right angles to the run-through axis of the workpiece, the drives for all the swinging jaws being so coupled together that all of the jaws are movable in synchronism, each said swinging jaw having a working surface with a portion of radius of curvature in each cross-section at right angles to the run-through axis of the workpiece equal to the distance between the centre of the sizing gap and the corresponding point on the working surface at that cross-section, and the edge region of the working surface on each side of the radiused portion of the working surface being flattened in the region of the deforming part. 